The number of terminals of an integrated circuit (IC) chip has been increasing. Therefore, recent IC chips are mounted on land grid array (LGA) substrates or ball grid array (BGA) substrates. In a LGA substrate, pads in a reticular pattern are arranged on its back. In a BGA substrate, pads in a reticular pattern are arranged on its back and spherical solder (referred to below as solder balls) is jointed to the pads.
Japanese Laid-open Patent Publication No. 10-12990 is an example of related art.
An IC chip having solder-ball-jointed pads (referred to below as chip pads) arranged in a reticular pattern is mounted on a LGA substrate or BGA substrate (referred to below as a BGA substrate or the like).
The pads (referred to below as substrate pads) corresponding to chip pads are arranged in a reticular pattern on a BGA substrate or the like. The IC chip is placed on the BGA substrate or the like so that the chip pads are aligned with the substrate pads, and then it is connected to the BGA substrate or the like through the reflow process.
The BGA substrate or the like includes a planar insulating substrate, substrate pads formed on its surface, external pads formed on its back, and traces that penetrate through the insulating substrate and connect the substrate pads and the external pads to each other.
The thermal expansion coefficient of the material (for example, epoxy resin) of the insulating substrate greatly differs from that of the material (for example, copper) of traces. The difference in the thermal expansion coefficient bends the BGA substrate or the like and its surface becomes uneven.
Accordingly, if the IC chip is placed on the BGA substrate or the like, only some solid balls make contact with the substrate pads and gaps are generated between the other solder balls and the substrate pads.
The larger the BGA substrate or the like, the larger the bend and unevenness of the substrate. As a result, the gaps between solder balls and substrate pads are widened. The gaps between solder balls and substrate pads remain after the reflow process, thereby causing failed connection between some chip pads and substrate pads.